Quantum Cascade Laser Lights a Fire at Photonics West

R. Winn Hardin

Lighting a match with a laser doesn't seem like that big a deal, unless it's done at room temperature using far-IR light from a diode laser.
It was in such a fashion that the inventor of quantum cascade laser diode architecture, Federico Capasso of Lucent Technologies/Bell Labs, demonstrated his latest work at the 1998 Photonics West conference. Last year, he reported a quantum cascade laser that emitted at 8 µm and produced 750 mW of CW power at liquid nitrogen temperatures. At that time, lasing output ceased at temperatures around 100 K.
With the help of lab partner Claire Gmachl, Capasso reported that they had produced 150 mW of pulsed power at room temperature at the same wavelength. Gmachl increased the injection efficiency by using two injection quantum wells to act as bookends to the AlInAs(InP) active region, while lowering the doped levels of the semiconductor material. The result was a reduction in threshold and better performance at higher temperatures.
But "we're never going to get the thresholds of well or dye lasers," Capasso said. "We're never going to get to the 100 amps per square centimeter." Gmachl's work required thresholds of 1 kA per square centimeter. Also, he said, peak CW output still requires cooling. Gmachl's device produced 200 mW of CW power at 80 K.

Engineering a wavelength
Quantum cascade lasers have generated between 1 and 13 µm through superlattice architectures. These designs produce different wavelengths by varying the thickness of many layers of the GaInAs/AlInAs(InP) semiconductor material, instead of seeking new bandgap materials. The cascade effect, or electron movement between subbands, creates between 25 and 30 photons per traveling electron, generating flexible diode architectures with very high powers compared with conventional devices. A large part of the success of quantum cascade design comes from material improvements. "Every step in material quality brings better results," Capasso explained.
The physicist expressed interest in other intersubband diode laser work being done around the world, such as that at Thomson-CSF and at several German universities where hybrid P-N/quantum cascade designs could lead to wavelengths outside the IR spectrum.